toaster: Fix oe-core fixture

toaster: Fix oe-core fixture

Due to a copy paste error we managed to get some of the wrong
information in the oe fixture that provides a suggested default settings
for Toaster. This meant it tested correctly when it shouldn't have.
Fix:
 - The use of local bitbake
 - An incorrect call to realpath which didn't include its parent module.
 - The field used for the local_dir of an existing openembedded-core

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>